Development of a novel metastable beta titanium alloy with ultrahigh yield strength and good ductility based on laser power bed fusion

Abstract

Metastable β titanium alloys are known for excellent ductility and corrosion resistance but relatively low yield strength (YS) which restricts their application. In this study, a novel high strength metastable β titanium alloy, Ti-6Mo-5.5Cr-1Co-0.1C (wt%) was developed based on laser powder bed fusion ( L -PBF). It is shown that the as-fabricated material was dominated by mixed columnar and equiaxed β grains decorated by numerous fine TiC flakes. The precipitates were interconnected to each other and connected to grain boundaries (GBs) by dislocations. Solution treatment at 850 °C led to dissolution of intra-granular TiC and diffusion of C to GBs through dislocation pipes, which created vast carbide-free β grain interior areas that are good for deformation and promoted formation of bulk and polygonal TiC along GBs. The GB TiC particles acted as effective O getter of the matrix, which together with significant ω precipitation helped stabilize the β matrix. As a result, β → α′′ phase transformation and mechanical twinning have been suppressed. The samples deformed mainly through dislocation slip and ω → β reverse phase transformation. Ultrahigh YS and decent ductility have thus been achieved in the newly developed metastable β titanium alloy. The current work paves the way for developing high performance metastable β titanium alloys through additive manufacturing.